Looks like you want to compute the Hamming distance (number of character positions in which they disagree) between two words. Do you really really need a regex, or will another way of matching be ok?

If you're not strongly committed to using a regex, use something like this (probably will do strange things for non-ascii character sets): 

sub hamming {
  my ($x1, $x2) = @_;
  return -1 if length($x1) != length($x2);

  (my $xor = $x1 ^ $x2) =~ tr/\x0//cd;
}

print hamming(@$_), $/
   for [qw[ abcdef abccef ]],
       [qw[ abcdef abc ]],
       [qw[ abcdef abbbbf ]];
__END__
1
-1
3

[download]
However, you could still cram this hamming function inside of a regex, with some trickery/cheating: 
# untested
my $target = "abcdef";
my $distance = 1;

my $len = length $target;
my $qr  = qr/ \b(\w{$target})\b (?(??{ hamming($target,$1) < $distance
+ }) | (?!))  /x

[download]
I'm in a bit of a rush at the moment, so my (?(cond)pattern) syntax is probably wrong. Also, it could be optimized greatly to avoid backtracking (say, with (?>pattern) to capture $1). Maybe someone can help me out here with the details, but this should give some idea..

blokhead

In reply to Re: searching for a string w/ a * in any single position? by blokhead
in thread searching for a string w/ a * in any single position? by mdunnbass

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.